Figure 2-2.—Distance measuring round-trip travel time.
second, per airborne equipment, to supply the
necessary distance data. However, the total pulse
out put of the transmitter constantly varies, according
to the number of interrogating aircraft. In addition,
random noise may trigger the transmitter.
Figure 2-3.—TACAN pulse train.
For the transponder to provide azimuth
information, the average power supplied to the
antenna must be relatively uniform over time. To
accomplish this, the transponder is operated on the
constant-duty-cycle principle.
In this method of operation, the receiver uses
automatic gain and squitter (noise generated output)
controls to maintain a constant pulse output to the
transmitter, as shown in figure 2-4.
If few
interrogations are being received, the gain and squitter
of the receiver increase and add noise-generated pulses
to the pulse train. If more interrogating aircraft come
into range, the gain and squitter decrease and reduce
the number of noise-generated pulses.
The relationship between the gain and the number
of pulses is such that only a 2-dBm change in
sensitivity occurs between reception from 1 aircraft
and those from 100 aircraft. An added advantage of
using a constant duty cycle is that overall transmitter
power drain remains constant.
BEACON-TRANSPONDER
IDENTIFICATION CODE
Before an aircrew can use TACAN information
that its equipment receives, it must positively identify
the transmitting TACAN station. To meet this need,
the ground station transmits an identification code at
approximately one-half minute intervals. It does this
by momentarily interrupting the transponder distance
data and squitter-generated output with pulse groups
spaced at a 1350-pps rate. Each pulse group contains
two sets of 12-µsec pulse pairs spaced 100 µsec apart.
The duration of the identification pulse groups varies,
to represent Morse-coded characters. The duration
for a dot is 100 to 125 ms, and for a dash 300 to 375
ms. An identification group is shown in figure 2-4.
2-2
